I’d like to take a stab at portions of this…
There are numerous reasons why season affects efficiency. One you already pointed out is the difference in formulation of the fuel during winter and summer months (more light, volatile aromatics in the winter for better vaporization, less in the summer because the heat makes the fuel vaporize nicely), as pointed out once by Rob Rapier. He has something on his blog that really describes this quite well. I think this is where Rob describes the winter formulation, but I’m not sure if that was the best one.
In another part of the thread, you also correctly state that the engine burns a richer fuel mixture as it’s trying to get to operating temperature range. Kind of funny that people jump in cars, drive them until they just reach the temp they were designed to run at, and then shut them off every day. No wonder people don’t reach the EPA listed ratings on their cars. BTW, I have a friend with a Prius that tells me that the engine always runs if you have the heater on because the heater gets its heat from the engine. Normally a Prius engine only kicks on when needed, so your fuel economy is substantially worse in the real world if you run the heater. You also correctly pointed out at one point in this thread that all of your engine and drivetrain lubricants (transmission fluid, differential, bearing grease, etc.) are more sticky and more viscous when they are cold.
Another factor is that the rubber in your tires is more rigid and has higher rolling resistance, which is greatly exacerbated if you don’t keep them properly inflated because the "bump" (deformation from the weight of the vehicle) you are pushing around the circle is bigger and more "steep" if the tire isn’t kept properly inflated.
Cold air is more dense, which is good for your engine, because you can pack more of it into the engine more easily for efficient combustion, but you also have to push through it, which has a tiny effect on aerodynamics. Warmer air is less dense, and thus you can glide through it a tiny bit easier, but I would have to do some back-of-napkin analysis to see if this is anywhere near significant, like it is for aircraft. This is why many aircraft fly at the altitudes they do, and why an F-15 can fly mach 2.5+ at optimal altitude, but just over mach 1 at sea-level—it has to push through denser air. At very high altitude, you don’t have a thick atmosphere to plow through, but your engines are working harder to pull in oxygen to produce power too. Cold air also improves the potential efficiency of a Carnot Heat Engine (a theoretical engine model predicting how efficient you can convert heat to work based on the max and minimum heat). The Carnot cycle is (TH – TC)/TH100%, where T is measured in Kelvins. The TC is the ambient air temp, and the TH is the max temp of your heat source (in this case the peak combustion temp in your cylinder). Theoretically your engine should operate more efficient thermodynamically, but in the real world the difference between summer and winter ambient air temps is small compared to the difference between them and the combustion temp, and the increased in theoretical efficiency is more than offset by the aforementioned fuel reformation for the winter months, the extra engine and drivetrain resistance and friction, etc. This is partly why your snorkels, ram air, and heat shields really help, is because you get the cooler, denser air into the engine (both because of the air density and temp) and it can expand a bit more when it’s heated. Boyles Law is also instructive to understand how the gases interact (the relationship between pressure, temperature and volume of a gas). PV=constant where temperature is held constant, V=cT where pressure is kept constant, and PV=nRT for an ideal gas.
As for compact cars, it’s weird, but US companies seem to equate small cars with "cheap", or people who don’t give a crap about quality, they just want "cheap". Ride in a Prius, or a Volkswagon or other small car and some countries’ design philosophies seem to understand that some people are willing to pay for quality, but don’t want a monster car. US automakers seem to assume that the more affluent you are, the more expensive car you want, the bigger you want it to be. I just don’t get why they can’t figure out that some people perceive value in small cars built with quality.
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